1. Field of the Invention
This invention relates to a modulation and demodulation system which modulates and demodulates a signal in a main channel for main data and a secondary channel for secondary data obtained by frequency division, and more particularly to a modulation and demodulation system of the type mentioned which uses, when it performs demodulation processing of main data, a frequency timing extracted from the secondary channel as a sampling timing for analog to digital conversion means for the main channel.
2. Description of the Related Art
Modems (modulator-demodulators) are generally used widely for transmission of data in analog circuits which make use of a voice band.
Some modems make use, in addition to a main channel for transmitting main data of a terminal or a like element, of a secondary channel provided by frequency division for transmitting signal quality information, a receive level and some other information to supervise the state of the network.
FIG. 15 shows a general construction of an on-line system. Referring to FIG. 15, in the on-line system shown, a plurality of modems 203 are connected to a host computer 201 by way of a communication control apparatus (CCP) 202, and each of the modems 203 is connected by way of transmission lines 204 to other modems 203' installed at another location. A terminal 205 is connected to each of the models 203'.
The on-line system further includes a network supervisory apparatus 206, for which a secondary channel is used.
By the way, a state signal of a modem can be transmitted, from each of the host side modems 203 shown in FIG. 15, as it is to the network supervisory apparatus 206, but from each of the terminal side modems 203', a state signal thereof is transmitted to the associated host side modem 203 so that it is transmitted by way of the modem 203 to the network supervisory apparatus 206.
Since a state signal of a modem must necessarily be transmitted without having any influence on main data, each of the modems 203 and 203' divides, for example, a voice band of 0.3 kHz to 3.4 kHz by frequency division to provide a secondary channel for secondary data in addition to a main channel for main data as seen in FIG. 16.
It is to be noted that phase shift keying (PSK), orthogonal amplitude modulation (QAM) or some other modulation is used for a main signal while frequency shift keying (FSK) is used for a secondary signal.
By the way, it is required for modems in recent years to establish multiple point connection in addition to a rise of the communication rate to reduce the cost of the circuit. To this end, it is an effective technique to divide a frequency band of a main channel into a plurality of bands to transmit a plurality Of data by way of the same circuit. An apparatus which is used for modulation and demodulation of a signal communicated in a main channel having a plurality of frequency bands obtained by frequency division is called multiple frequency modem.
When the main channel involves a single frequency band, the roll-off ratio is sufficiently high to extract a timing for use for demodulation processing of main data readily from the main channel. However, when the main channel is divided into a plurality of frequency bands as described above, the roll-off ratio of the main channel is decreased so low that it is difficult to extract a timing component from the main channel.
Therefore, it is necessary to extract a timing component from the secondary channel whose roll-off ratio is sufficiently high. Thus, a multiple frequency modem uses, when it performs demodulation processing of main data, a frequency timing extracted from the secondary channel as a sampling timing for a digital value (output of an analog to digital converter) of the main data.
When the frequency band of the main channel is not divided, a frequency timing extracted from the main channel can be used as it is as a sampling timing for a digital value as described above, and accordingly, the timings coincide with each other without fail and no timing error occurs.
With a multiple frequency modem described above, however, since a digital value (output of an analog to digital converter) is sampled with a frequency timing extracted from the secondary channel whereas a phase timing extracted from the main channel is used for demodulation processing of main data, a timing error (an out-of-synchronization condition) sometimes occurs only in demodulation processing of main data. Accordingly. it is desired that, when such a timing error actually occurs, the modem itself restores demodulation processing of the main channel based on a normal timing.